This invention relates to an apparatus for gravity feeding balls to a grinding mill. More particularly, the invention relates to an apparatus for sequentially feeding grinding balls one-at-a-time to a grinding mill at a controlled rate.
Grinding mills are used to reduce the size of solid materials in tumbling mills during mineral processing of ores. The size reduction is accomplished by the tumbling action of grinding media during rotation of the mill. Grinding media may be spherically shaped such as balls, non-spherically shaped such as cylinders or cones or some combination thereof. Traditionally, grinding media is intermittently fed to a mill using a dump bucket and the like. More recently, automatic feeders have been used with the feeders supplying controlled amounts of grinding media at timed intervals. U.S. Pat. No. 4,715,546 discloses an apparatus for storing and feeding multiple balls of limited sizes to a grinding mill. A drum having small compartments is positioned in-line with a downwardly inclined chute. As the drum is continuously rotated at a preset speed, an empty compartment on the upstream side of the drum is filled with balls from the chute while another compartment on the downstream side of the drum discharges a similar amount of balls into the grinding mill. Rotating feeders have a disadvantage that ball pieces tend to become wedged between the rotating feeder and the inclined chute. A jammed ball piece causes the feeder to lock and the jammed piece can not be freed by rotating the feeder in the reverse direction. Further, if the jammed ball piece is inaccessible, the rotating feeder must be disassembled thereby causing a unit stoppage.
Rotating or star feeders also are limited to the ball sizes that can be used because the ball compartments have a specific size. Balls too small for a specific compartment tend to bridge in the compartments, in effect clogging the compartments. This changes the addition rate since fewer balls are fed during each revolution. On the other hand, balls that are too large in diameter will not fit into the compartments thus causing the feeder either to slip against the balls in the chute or to lock in place and possibly burn out the feeder motor. Thus, various size star feeder assemblies must necessarily be manufactured and used according to the particular ball sizes being used.
Changing the size of a rotating feeder assembly also may require the width of the inclined chute to be changed. Because multiple balls are fed from the storage bin to the feeder assembly, the balls will be positioned side-by-side in the chute. This side-by-side positioning may cause the balls to bridge in the chute when the ball diameter ratio changes significantly. That is, the likelihood of balls jamming in the chute increases as the ball diameter to chute width ratio increases.
Russian patent 216,428 discloses a device for feeding grinding balls. The device includes a pneumatically operated cylinder having an upper cover adapted for receiving balls, a downwardly inclined first chute filled with balls and a downwardly inclined second chute for delivering balls to a grinding mill. The pneumatic cylinder is passed upwardly through the first chute picking up balls with the balls then being transferred to the second chute. This ball feeder has the disadvantages that balls are not fed one-at-a-time, fed balls are not monitored and balls tend to jam as the first chute is raised. If a jammed condition does occur, the potential for damage is increased because the pneumatic cylinder will continue attempting to raise the first chute causing the pressure in the cylinder to continue increasing until failure occurs.
Accordingly, there remains a need for an apparatus for feeding grinding balls into a grinding mill wherein the potential for jamming by the balls or the feeder mechanism is minimized. There also remains a need for an apparatus that can monitor and control the feed rate of grinding balls.